This page allows you to enable or disable specific installed decoders, as well as specify which one takes precedence if multiple decoders can open the same file.
Please note that this page has very little effect for most foobar2000 configurations. It becomes meaningful with specific add-on components installed. Changing the decoder order has no impact on app performance, it's only meant to let you resolve conflicts between decoders.
This is a new feature in foobar2000 v1.4. In older versions of foobar2000, the order in which decoders were probed was random by design.
You might see an "unnamed legacy decoders" entry in the list - identification and naming of decoders was introduced in foobar2000 v1.4 and older components may not supply identification info allowing foobar2000 to remember their status. In order to correct this, the component needs to be rebuilt with a newer foobar2000 SDK and supply the new methods.
Some decoders provide a configuration page - you will see a blue "configure" link next to their names.